How would you like to be remembered? Philomena McGuinness knows the answer to that: she’s a great Irish poet, in the mould of her hero W B Yeats. Or at least, she’s destined to become one. In the meantime she’s a 19-year-old nurse, who’s crossed the Irish Sea to make her living in London; but it’s 1939, the storm clouds of war are gathering in Europe, and Philomena won’t be the only one to find a shocking new reality intruding on her dreams.

Philomena isn’t really a great poet. She isn’t a ‘great’ nurse either, in the sense of Florence Nightingale, though she’s a caring and compassionate one. She signed up to goad her mother, who claimed she was too ‘flighty’ – a description she simultaneously resents, and seems to wear with pride. She conforms just enough, rebels just enough, and writes just enough poetry to keep the flame of her ambition alive.

Jasmin Gleeson plays Philomena as likeable and free-spirited, shaking off the last of youthful uncertainty but often wise beyond her years. She has a healthy respect for whiskey – both its ‘healing power’ and its closeness to the void – and she’s given to cheeky asides and forays into profanity. As we hear about her unexpected adventures in Blitz-torn London and at a camp for evacuees, it’s a credible and entertaining portrayal of an Irishwoman, among Brits, at time of war.

But we know, even before she tells us, that her chatter is a coping mechanism – a way of pretending that things are normal, for fear that if she pauses too long ‘reality will catch up’ with her. And when the war does catch up, as we know it will, Gleeson’s performance elevates from the captivating to the sublime. Motifs carefully planted in those carefree earlier scenes return with new meaning; and though Philomena tries to stay bubbly, there’s a new brittleness underneath it all. She rarely speaks of the horror she witnesses, but we always can tell that it’s there.

If I’ve a criticism, it’s that the high-speed delivery – though entirely in character – can make the narrative a little hard to hold onto. (So much so, in fact, that Philomena got deployed to the front line without my really noticing, until soldiers with shredded bodies began to appear.) The pace does ease up as the tone grows more wistful, but a suspicion remains that the early part is rushed because they’re trying to cram just a little too much in.

But that’s a minor footnote to a deeply affecting performance, which does full justice to Joshua King’s masterpiece script. This is a play that speaks of big themes: of defining yourself before others define you, and of catching the moment because you never know what the future holds. And there are simpler points too, about comradeship in adversity, and the bittersweet truth that we sometimes burn brightest in the darkest of times. And so – while she may not be another Yeats – I’m sure I’ll remember Philomena for a long time to come.